1. Field of the Invention
The present invention relates to an aperture and an optical device using it, and more particularly, the present invention aims at improving a structure of the aperture and thus a function which is required for the optical device.
2. Description of the Background Art
Recently, various optical devices have been used for manufacturing of a semiconductor device. A structure of these optical devices includes a light source for emitting a light beam, a collimator lens for making a light beam parallel, an aperture for shaping a light beam passing through the collimator lens into a prescribed shape, and an objective lens for condensing a light beam passing through the aperture.
An optical device with the above-described structure includes, for example, a photomask defect repairing apparatus for repairing a defect found in a photomask, and a scanning critical dimension measurement apparatus for measuring the dimension of a pattern which is formed on a semiconductor substrate. The photomask defect repairing apparatus for repairing a defect which has been found in a photomask will now be described.;
Referring to FIG. 11, in a photomask 200, a metal thin film 208 which includes a light-transmitting portion 206 and a light-shielding portion 204 is formed on a transparent glass substrate 202. About 10 to 20 kinds of photomasks 200 each of which has a different pattern are required to manufacture a single semiconductor device. If a defect exists in these photomasks, the defect also is transferred to a wafer, resulting in poor quality and reduced yield of the semiconductor device.
A defect which could be found in photomask 200 can be classified into two types: a remaining defect 210 and a pinhole defect 220. In order to repair pinhole defect 220, a carbon-type film 224 is usually deposited and filled in pinhole defect portion 220 by an FIB assist deposition system.
In order to repair remaining defect portion 210, a laser beam 222 is collected and directed to remaining defect portion 210 and the metal thin film is removed. A device using YAG (Yttrium, Aluminum, Gahnite) laser has been practically available. Higher resolution has been achieved by use of second harmonics (wavelength of 0.53 xcexcm) of YAG laser, and a mask defect repairing apparatus using the second harmonics, therefore, has been used increasingly.
The basic structure of this mask defect repairing apparatus is disclosed in Japanese Patent Laying-Open No. 60-132325. Referring to FIG. 12, the structure of the mask defect repairing apparatus 250 which has been disclosed in Japanese Patent Laying-Open No.60-132325 includes a light source (Nd. YAG laser) 252 for emitting a light beam 254, a collimator lens 256 for making light beam 254 parallel, a variable rectangular aperture 258 for shaping laser beam 254 passing through collimator lens 256 into a prescribed shape, and an objective lens 260 for condensing laser beam 254 passing through aperture 258 into a defective portion 264 on a photomask 262.
Referring to FIG. 13, aperture 258 used in this mask defect repairing apparatus 250 is constituted by an opening 268 for shaping laser beam 254 into a prescribed rectangular shape, and a light-shielding portion 266 for intercepting a laser beam.
The structure of a critical dimension measurement apparatus 300 will now be described with reference to FIG. 14. Critical dimension measurement apparatus 300 includes a light source 302 for emitting a laser beam 301a, a collimator lens 304 for making laser beam 301a parallel laser beam 301b, an aperture 306 for shaping laser beam 301b into a laser beam 301c having a prescribed rectangular shape, and an objective lens 310 for condensing laser beam 301c into a surface of a wafer 314. This wafer 314 includes a pattern 314a with a prescribed shape, which is made of conductive material, insulating material or the like, formed on a substrate 314b. A laser beam is reflected from wafer 314 and then from a semi-transparent mirror 308, and enters a detector 312.
Width d of pattern 314a is measured, referring, for example, to FIG. 15, when laser beam 301d is moved with respect to pattern 314a in the direction shown by an arrow and change in intensity between light beams 301d reflected from substrate 314b and from pattern 314a is recognized by detector 312.
Principle of measurement of width d of pattern 314a will now be described with reference to FIG. 16. In FIG. 16, abscissa indicates a ratio of half-width W of a scanning beam (laser beam) to width d of a pattern, and ordinate indicates intensity of a signal indicated by a reflected light beam which is received by a detector. Resolution of the critical dimension measurement apparatus will now be described with reference to FIG. 16. Resolution is usually represented by a W/d value that is obtained just before the decrease of intensity of a reflected light beam. For example, pattern width d can be measured, if two edge portions of the pattern can be recognized. Therefore, as understood from FIG. 16, pattern width d can be measured so long as the W/d value is 2.0 or less. This is explained in more detail with reference to FIGS. 17-21.
FIG. 17 shows change in signal intensity when a scanning beam moves across a substrate and W/d is 0.5. By representing intensities of signals received from substrate 314b and pattern 314a as A and B (A greater than B), respectively, the signal intensity becomes (A+B)/2 when the central point of the scanning beam comes to an edge portion of pattern 314a. Therefore, pattern width d can be measured if signal intensity of (A+B)/2 can be recognized.
FIG. 18 shows change in signal intensity when a scanning beam moves across a substrate and W/d is 1.0. In this case, pattern width d can be measured by recognizing signal intensity of (A+B)/2, as shown in FIG. 17.
FIG. 19 shows change in signal intensity when a scanning beam moves across a substrate and W/d is 2.0. In this case, pattern width d can be easily measured, since the range in which signal intensity is (A+B)/2 will correspond to pattern width d.
FIG. 20 shows change in signal intensity when a scanning beam moves across a substrate and W/d is 2.5. In this case, the range in which signal intensity is (A+B)/2 cannot be recognized. Therefore, pattern width d cannot be measured.
From above description, it is to be understood that patten width d cannot be measured when W/d is more than 2.
Recently, with miniaturization of a semiconductor device, improvement in degree of integration and in function of the semiconductor device has been desired, resulting in requirement of further miniaturization of an element which is formed in a semiconductor device. Therefore, optical devices such as a pattern defect repairing apparatus and a critical dimension measurement apparatus which can deal with the miniaturization have been required.
In these optical devices, spot, that is, half-width of a laser beam must be reduced in order to deal with miniaturization of a pattern. Reduction in half-width can be achieved by reducing an opening width of an aperture. Half-width can be reduced when the opening width of the aperture is larger than a wavelength of light, while diffraction of light might introduce a problem when the opening width of the aperture becomes smaller with miniaturization of a pattern. This is illustrated in FIGS. 21A to 21D. Light has such a clear amplitude as shown in FIG. 21B right after passing through the aperture. Light projected onto a wafer, however, has an amplitude that is widened at its bottom, such as shown in FIG. 21C, because of diffraction of light. Therefore, light on the wafer has an intensity which is not sharp as shown in FIG. 21D, resulting in increase in half-width.
It is known that light passing through the aperture has the corners rounded with radius of curvature of about the wavelength of light because of the above-described diffraction. When a laser beam, for example, of 0.53 xcexcm is used, the minimum half-width of the laser beam will be 1.06 xcexcm. Therefore, in repairing a pattern defect, it is very difficult to repair with high accuracy a remaining defect in a pattern of 1 xcexcm wide.
In the critical dimension measurement apparatus, when half-width is 1.06 xcexcm, pattern width of less than 0.53 xcexcm cannot be measured, since the above-described W/d becomes larger than 2.0.
It is an object of the present invention to provide an aperture by which reduction in diffraction of light passing through the aperture and in half-width of light can be achieved, and to provide an optical apparatus using this aperture.
An aperture in accordance with one aspect of the present invention is intended to be used in an optical device and includes a first light-transmitting region and a second light-transmitting region.
The first light-transmitting region transmits light emitted from a light source which is provided in the optical device. The second light-transmitting region is provided in the periphery of the first light-transmitting region and provides light passing therethrough with a phase difference of 180xc2x0 with respect to light passing through the first light-transmitting region.
Thus, since part of light passing through the first light-transmitting region, in particular, through the peripheral edge portion thereof is offset by light passing through the second light-transmitting region and having a phase difference of 180xc2x0, increase in half-width due to diffraction of light passing through the first light transmitting region can be prevented. Accordingly, half-width of light passing through the first light-transmitting region can be reduced, resulting in a clear optical image.
An aperture in accordance with another aspect of the present invention is intended to be used in an optical device and includes a phase shift substrate which has a first opening with a prescribed shape and provides light passing therethrough with a phase,difference of 180xc2x0, and a light-shielding substrate which has a second opening similar to and larger than the first opening and intercepts light. The phase shift substrate and the light-shielding substrate are positioned so that the first opening is surrounded by the second opening.
Preferably, the phase shift substrate and the light-shielding substrate are positioned on a transparent substrate which transmits light.
More preferably, the phase shift substrate is made of glass which is selected from the group consisting of oxide glass, soda-lime glass, borosilicate glass, lead glass, alumina-silicate glass, borate glass, phosphate glass, aluminate glass, titanate glass, fluoride glass, chalcogenide glass, metal glass, and crystallized glass.
This aperture includes a first opening through which light passes, and a light-shielding substrate having such a second opening that exposes only the periphery of the first opening of a phase shift substrate which provides light passing therethrough with a phase difference of 180xc2x0.
Thus, since part of light passing through the first opening, in particular, through the peripheral edge portion thereof is offset by light of which phase is shifted by 180xc2x00 through the phase shift substrate which is exposed by the second opening in the periphery of the first opening, increase in half-width due to diffraction of light passing through the first opening can be prevented. Accordingly, half-width of light passing through the first opening can be reduced, resulting in a clear optical image.
An aperture in accordance with further aspect of the present invention is intended to be used in an optical device, and includes a phase shift substrate and a light-shielding substrate.
The phase shift substrate includes a first light-transmitting portion with a prescribed thickness, and a second light-transmitting portion which is thicker than the first light-transmitting portion and provides light passing therethrough with a phase difference of 180xc2x0 with respect to light passing through the first light-transmitting portion.
The light-shielding substrate exposes only the first light-transmitting portion and a prescribed region of the second light-transmitting portion, which is located in the periphery of the first light-transmitting portion, and shields the other region thereof from light.
In this aperture, since part of light passing through the first light-transmitting portion, in particular, through the peripheral edge portion thereof is offset by light passing through the second:light-transmitting portion and having a phase difference of 180xc2x0, increase in half-width due to diffraction of light passing through the first light-transmitting portion can be prevented. Accordingly, half-width of light passing through the first light-transmitting portion can be reduced, resulting in a clear optical image.
An optical device in accordance with the present invention includes an aperture which has a second light-transmitting region which proves light passing therethrough with a phase difference of 180xc2x0 with respect to light passing through the periphery of a first light-transmitting region.
Thus, since part of light passing through the first light-transmitting region, in particular, through the peripheral edge portion thereof is offset by light passing through the second light-transmitting portion and having a phase difference of 180xc2x0, increase in half-width due to diffraction of light passing through the first light-transmitting region can be prevented. Accordingly, reduction in half-width of light passing through the first light-transmitting region can be achieved.
As a result, a function which is required for an optical device can be improved, and, for example, a pattern defect repairing apparatus will be able to repair a defect in a finer pattern, and a critical dimension measurement apparatus will be able to measure the dimension of a finer pattern.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.